The present invention relates to a uniaxially oriented film formed from a film-forming material comprising a polypropylene-based resin material. More precisely, it relates to a uniaxially oriented polypropylene-based film in which tensile elongation along the transverse direction is improved without degrading transparence.
Uniaxially oriented polypropylene-based films have a property that they are likely to be torn along the film-flowing direction during the film production, i.e., the longitudinal direction called machine direction (MD direction), and thus they are excellent in cuttability along that direction (straight cuttability). Therefore, they are widely used mainly in the field of food packaging, for example, individual packaging of products likely to break, such as confectionery, individual packaging of rice balls and the like.
While conventional uniaxially oriented polypropylene-based films are excellent in the straight cuttability along the MD direction, however, they are poor in elongation along the perpendicular direction to the MD direction (TD direction), and therefore they are likely to be torn when force is applied along the TD direction. Therefore, the films may be broken when used for packaging of heavy contents.
Further, when the conventional uniaxially oriented polypropylene-based films are torn along the MD direction, they generate fine fibers on their torn surfaces, which may be mixed in foodstuffs and the like.
The object of the present invention is to provide a uniaxially oriented polypropylene-based film in which tensile elongation along the TD direction is improved without degrading transparence of the film, and which generates less fibers when it is torn.
The present inventors earnestly conducted studies in order to achieve the foregoing object. As a result, they found that, in a uniaxially oriented film composed of a polypropylene-based resin material comprising a propylene-xcex1-olefin copolymer dispersed as particles in crystalline polypropylene, the straight cuttability along the MD direction and the tensile elongation along the TD direction could be improved without degrading the transparence by controlling form of the film so that the film should have a specific dispersion state of the copolymer particles in a cross section along the MD direction of the film. Thus, they accomplished the present invention.
That is, the present invention provides a uniaxially oriented polypropylene-based film formed from a film-forming material which comprises a polypropylene-based resin material consisting of 40 to 95% by weight of crystalline polypropylene, and 60 to 5% by weight of propylene-xcex1-olefin copolymer dispersed as particles in the crystalline polypropylene, wherein the particles of the copolymer have an aspect ratio (L/D) of mean dispersed particle length (L) to mean dispersed particle diameter along the film thickness direction (D) of 100 or more in a cross section along the MD direction of the film, and the mean dispersed particle diameter of 0.10 xcexcm or less.
The present invention also provides a uniaxially oriented polypropylene-based film formed from a film-forming material which comprises a polypropylene-based resin material consisting of 40 to 95% by weight of crystalline polypropylene, and 60 to 5% by weight of propylene-xcex1-olefin copolymer, and has a ratio of MFR of the crystalline polypropylene to that of the propylene-xcex1-olefin copolymer (MFR of the crystalline polypropylene/MFR of the propylene-xcex1-olefin copolymer) of 10 or less, wherein the film is uniaxially oriented along the MD direction so that the orienting ratio should be 3 to 12 times.
The uniaxially oriented film of the present invention comprises the copolymer particles dispersed elongatedly in a matrix of the crystalline polypropylene in such a manner the particles should have an aspect ratio higher than a certain level, and an oriented film composed of a polyolefin-based resin material having such a dispersion state has been made by the present invention for the first time.
According to the present invention, there can be obtained an oriented film that exhibits good straight cuttability along the MD direction, excellent tensile elongation along the TD direction and high transparence, and does not generate fibers, thanks to such a characteristic in form.
The production method of the film is not particularly limited so long as an oriented film having such a characteristic in form as mentioned above can be provided. However, a uniaxially oriented polypropylene-based film which is composed of a polypropylene-based resin material comprising crystalline polypropylene and propylene-xcex1-olefin copolymer and having a ratio of MFR of the crystalline polypropylene to that of the propylene-xcex1-olefin copolymer of 10 or less, and oriented under a certain condition can have the aforementioned characteristic.
The uniaxially oriented film of the present invention is useful as a film for package, in particular, as a film for packaging heavy contents or for food package.
Preferred embodiments of the present invention will be explained hereinafter.
The film-forming material for forming the uniaxially oriented film of the present invention comprises a polypropylene-based resin material that consists of crystalline polypropylene and propylene-xcex1-olefin copolymer, the copolymer being dispersed as particles in the crystalline polypropylene (the copolymer is dispersed as domains in a matrix of the crystalline polypropylene).
(i) Crystalline polypropylene
The crystalline polypropylene used for the present invention is a crystalline polymer comprising principally of propylene units, and preferably comprises 90% by weight or more of the propylene units based on the whole polymer. Specifically, it may be a homopolymer of propylene, or it may be a random copolymer or a block copolymer comprising 90% by weight or more of propylene units and less than 10% by weight of xcex1-olefin. When it is a copolymer, the xcex1-olefin may include ethylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 4-methyl-1-pentene, 3-methyl-1-pentenel, and the like. It is preferable in view of the production cost to use a propylene homopolymer or propylene-ethylene copolymer having a propylene unit content of 90% by weight or more.
The melt flow rate (abbreviated as xe2x80x9cMFRxe2x80x9d hereinafter) of the crystalline polypropylene is preferably in the range of 0.1-50 g/10 minutes in view of the stability upon film-forming.
(ii) Propylene-xcex1-olefin copolymer
The propylene-xcex1-olefin copolymer used for the present invention is a random copolymer of propylene and an xcex1-olefin other than propylene. The content of propylene unit is preferably in the range of 20-80% by weight, more preferably 20-75% by weight, particularly preferably 20-70% by weight based on the whole copolymer. When the content of propylene unit exceeds 80%, the desired dispersed state of the copolymer particles (referred to as xe2x80x9ccopolymer domainsxe2x80x9d hereinafter) in the matrix of crystalline polypropylene may not be obtained, and hence the improvement of the tensile elongation along the TD direction targeted in the present invention may not be obtained. On the other hand, when it is less than 20%, the copolymer domains are difficult to be formed, and thus the desired performance may not be obtained.
As the xcex1-olefin other than propylene, ethylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 4-methyl-1-pentene, 3-methyl-1-pentene and the like can be mentioned. Among these, a propylene-ethylene copolymer containing ethylene as the xcex1-olefin is preferably used because it is beneficial to the production cost.
While MFR of the propylene-xcex1-olefin copolymer used for the present invention is not particularly limited, it is preferably in the range of 0.1-20 g/10 minutes.
More preferably, MFR of the propylene-xcex1-olefin copolymer is preferably selected so that its ratio to MFR of the crystalline polypropylene (MFR of the crystalline polypropylene/MFR of the propylene-xcex1-olefin copolymer, referred to as xe2x80x9cMFR ratioxe2x80x9d hereinafter) should be 10 or less, more preferably fall within the range of 0.1-5.
(iii) Polypropylene-based resin material
In the polypropylene-based resin material of the present invention, the content of the crystalline polypropylene is 40-95% by weight, preferably 50-95% by weight, and the content of the propylene-xcex1-olefin copolymer is 60-5% by weight, preferably 50-5% by weight based on the whole polypropylene-based resin material. When the ratio of the copolymer is less than 5% by weight, satisfactory tensile elongation along the TD direction cannot be obtained. When it exceeds 60%, the rigidity of the film is markedly decreased, and it is not preferred for practical use.
The production method of the polypropylene-based resin material is not particularly limited, and it can be obtained by any kind of method. For example, it can be obtained by mixing crystalline polypropylene and propylene-xcex1-olefin copolymer, which were polymerized separately, through melt-kneading or the like. Alternatively, it can be obtained by continuously polymerizing crystalline polypropylene and propylene-xcex1-olefin copolymer by multi-step polymerization.
Specifically, a method based on melt-kneading of propylene-xcex1-olefin copolymer polymerized by using a Ziegler-Natta catalyst such as titanium-supported catalyst or commercially available ethylene-propylene rubber and crystalline polypropylene can be exemplified. As the method for continuously polymerizing crystalline polypropylene and propylene-xcex1-olefin copolymer by multi-step polymerization, for example, a method comprising producing propylene homopolymer in the first step, and producing propylene-xcex1-olefin copolymer in the second step by utilizing a plurality of polymerization reactors can be exemplified. This continuous polymerization method is preferred, because it can be performed at a lower cost compared with the aforementioned melt-mixing method, and can produce a polypropylene-based resin material where the propylene-xcex1-olefin copolymer is uniformly dispersed in the crystalline polypropylene, and it is suitable for stably realizing the desired quality (tensile elongation along the TD direction, preventing the generation of fibers, good transparence).
As the polypropylene-based resin material of the present invention, particularly preferred are those produced by the aforementioned continuous polymerization method so that the resulting material should have the MFR ratio of the crystalline polypropylene to that of the propylene-xcex1-olefin copolymer (MFR of the crystalline polypropylene/MFR of the propylene-xcex1-olefin copolymer) of 10 or less, more preferably in the range of 0.1-5. By selecting the MFR ratio within this range, the propylene-xcex1-olefin copolymer can be uniformly and finely dispersed in the crystalline polypropylene, and the copolymer particles can have elongated form with an aspect ratio higher than a certain level after the uniaxial orienting. This provides a polypropylene-based oriented film that shows good straight cuttability along the MD direction, and tensile elongation along the TD direction further improved without degrading the transparence.
Specifically, polypropylene-based resin materials having such an MFR ratio can be produced by the methods mentioned in Japanese Patent Unexamined Publication Nos. 6-239918, 8-27238, and the like.
The MFR ratio can usually be calculated by measuring the MFR of the crystalline polypropylene and the MFR of the propylene-xcex1-olefin copolymer respectively, but when the polypropylene-based resin material is continuously produced by the multi-step polymerization method (the crystalline polypropylene is polymerized first, and then the propylene-xcex1-olefin copolymer is polymerized), the MFR of the propylene-xcex1-olefin copolymer cannot be directly measured. In such a case, the MFR of the propylene-xcex1-olefin copolymer can be obtained from the MFR of the crystalline polypropylene, which can be directly measured, the MFR of the obtained polypropylene-based resin material, and the content of the propylene-xcex1-olefin copolymer in the polypropylene-based resin material according to the following equation:       log    ⁡          (              MFR        RC            )        =                    log        ⁡                  (                      MFR            whole                    )                    -                        (                      1            -                                          W                RC                            ⁢                              /                            ⁢              100                                )                ⁢                  log          ⁡                      (                          MFR              PP                        )                                              W        RC            ⁢              /            ⁢      100      
MFRRC: MFR of propylene-xcex1-olefin copolymer
MFRwhole: MFR of polypropylene-based resin material
MFRPP: MFR of crystalline polypropylene
WRC: Content of propylene-xcex1-olefin copolymer in polypropylene-based resin material
While the film-forming material of the present invention is mainly composed of the aforementioned polypropylene-based resin material, it may further contain additives conventionally used for polyolefin-based film materials, for example, antioxidant, neutralizer, weathering agent, inorganic filler, lubricant, anti-blocking agents, antistatic agent and the like.
Examples of the antioxidant include, for example, phenol compound antioxidants such as tetrakis[methylene-3-(3xe2x80x2,5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxyphenyl)propionate]methane, 2,6-di-t-butyl-4-methylphenol, n-octadecyl-3-(3xe2x80x2,5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxyphenyl)propionat e, and tris(3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate; phosphorus-containing antioxidants such as tris(2,4-di-t-butylphenyl) phosphite, tris(nonylphenyl) phosphite, distearylpentaerythritol diphosphite, and tetrakis(2,4-di-t-butylphenyl)-4,4xe2x80x2-biphenylenediphosphonite, and the like.
Examples of the neutralizer include, for example, salts of higher fatty acid such as calcium stearate; examples of the inorganic filler and the anti-blocking agents include, for example, calcium carbonate, silica, hydrotalcite, zeolite, aluminum silicate, magnesium silicate and the like; examples of the lubricant include, for example, higher fatty acid amides such as stearic acid amide and the like; and examples of the antistatic agents include, for example, fatty acid esters such as glycerin monostearate and the like.
While the amounts of these additives may be suitably selected depending on the intended use of the film and the like, they are preferably used in an amount of about 0.001-5% based on the whole film-forming material in general.
The method for mixing the polypropylene-based resin material and the aforementioned additives is not particularly limited, and it can be performed, for example, by mixing methods utilizing conventional mixing apparatuses including mixers provided with high-speed agitators such as Henschel mixer (trade name), ribbon blender and tumbler mixer and the like (dry blend), as well as methods for pelletization utilizing a conventional single-screw extruder, double-screw extruder and the like.
The uniaxially oriented film of the present invention can be obtained by uniaxially orienting the aforementioned film-forming material. The orienting can be performed by molding an unoriented sheet by the known T-die cast method, water-cooled inflation method or the like, and then orienting it by a known uniaxial orienting method such as the roll orienting method.
While the orienting ratio along the MD direction (longitudinal orienting ratio) for the uniaxially oriented film of the present invention is not particularly limited, it is, for example, 3-12 times, preferably 5-10 times. An orienting ratio in such a range can afford an aspect ratio of 100 or more for the ratio of the mean dispersed particle length to the mean dispersed particle diameter along the film thickness direction in a cross section of the propylene-xcex1-olefin copolymer domains along the MD direction, and provide a film with improved tensile elongation without degrading the transparence.
In the uniaxially oriented polypropylene-based film of the present invention, the propylene-xcex1-olefin copolymer domains dispersed as particles in the crystalline polypropylene have a mean dispersed particle diameter along the film thickness direction of 0.10 xcexcm or less, preferably 0.05 xcexcm or less in a cross section along the MD direction. When the mean dispersed particle diameter is more than 0.10 xcexcm, the tensile elongation along the TD direction is lowered, and transparence is also degraded. On the other hand, the lower limit of the mean dispersed particle diameter is not particularly defined, and it may be however small so long as the copolymer domains can be observed. However, the mean dispersed particle diameter is preferably not less than 0.005 xcexcm.
The uniaxially oriented film of the present invention is also characterized by the aspect ratio (L/D) of 100 or more, preferably 300 or more, as for the ratio of the mean dispersed particle length (L) to the aforementioned mean dispersed particle diameter (D) of the copolymer domains in a cross section along the MD direction.
The relationship between the mean dispersed length (L) and the mean dispersed particle diameter (D) is schematically shown in FIGS. 1(a) and (b). The mean dispersed particle diameter (D) in a cross section along the MD direction is the average of the particle diameter (breadth) of the dispersed particles along the film thickness direction when the cross section of the film along the MD direction is observed from the perpendicular direction to the MD direction (MD observation: edge view). The mean dispersed length (L) is the average of the length of the dispersed particles in the aforementioned MD observation.
According to the present invention, such fine and elongated copolymer domains are uniformly dispersed in the matrix. This provides a uniaxially oriented film excellent in tensile elongation along the TD direction, and exhibiting transparence not degraded. An aspect ratio of less than 100 is not preferred, because the film with such an aspect ratio may degrade the tensile elongation along the TD direction and transparence.
Although the upper limit of the aspect ratio is not particularly limited, it is preferably about 700 when a length of one copolymer particle is regarded as the particle diameter along the MD direction of the copolymer domain. However, the copolymer particles may be fused and connected each other along the MD direction by the orienting along the MD direction. In this case, when the multiple fused copolymer particles is considered as one copolymer domain, the particle diameter along the MD direction may be several times as large as the length of one copolymer particle. The maximum aspect ratio of such a copolymer domain may be several times that of the individual one copolymer particle, specifically 10 to 50 times. In this case, the aspect ratio may reach as high as around 1000 to 5000.
When a cross section along the TD direction of the film of the present invention is observed from the perpendicular direction to the TD direction (TD observation: end view), the copolymer domains may be in a flattened form due to the uniaxial orienting along the MD direction. In such a case, the aspect ratio (Lxe2x80x2/D) of the mean dispersed length (Lxe2x80x2) to the mean dispersed particle diameter (D) along the film thickness direction in the cross section along the TD direction is preferably, while it is not particularly limited, about 80 to about 600. The TD observation is schematically shown in FIG. 1(c).
According to the present invention, it was found for the first time that a film containing the copolymer domains which exhibited such a fine mean dispersed particle diameter and such an aspect ratio as described above was excellent in the tensile elongation along the TD direction, and does not degrade transparence. Therefore, the film of the present invention may be a film obtained by any kind of method so long as the film satisfies the requirements concerning the particle diameter of the copolymer domains. However, it can specifically be obtained by orienting a polypropylene-based resin material produced by the above-mentioned continuous polymerization method.
Particularly preferably, the film can be obtained by uniaxially orienting a polypropylene-based resin material being produced by the continuous polymerization method and having a ratio 10 or less of MFR of the crystalline polypropylene to that of the propylene-xcex1-olefin copolymer to have an orienting ratio of about 3 to 12 times.
The thickness of the uniaxially oriented polypropylene-based film of the present invention is not particular limited, but it is preferably 10-100 xcexcm, more preferably 15-70 xcexcm, in view of the film-forming property of the film.
The uniaxially oriented polypropylene-based film of the present invention is excellent in the straight cuttability along the MD direction and the tensile elongation along the TD direction, does not generate fibers, and retains transparence. Therefore, it can be preferably used as a material for packaging heavy contents, material for packaging foodstuffs such as sandwiches and rice balls, and the like.
The uniaxially oriented polypropylene-based film of the present invention can also be used for a multilayer film comprising two or more layers, which can be prepared by laminating one or more films made of other resins on one or both sides of the film of the present invention. The other resins used for such a laminated film are not particularly limited, and various resins can be used depending on the purpose of the film. For example, when a layer composed of heat adhesive resin such as propylene-xcex1-olefin copolymer having a low melting point is provided on the uniaxially oriented film of the present invention, it can be used as various package materials. As production methods of such a multilayer film, the inline laminating method ,the co-extrusion method and the like, which are performed during the film molding, as well as the dry laminating method and the like where the lamination is performed after the film molding, can be utilized.